Many display devices use a layer of electro-optic material which changes at least one optical property when a suitable electric field is applied across it. Examples of electro-optic materials include nematic or smectic liquid crystals and electrophoretic materials. Liquid crystal displays (LCDs) typically comprise opposed planar substrates (cell walls) with electrodes on the inner surface of each wall. The substrates are spaced apart and enclose a layer of a liquid crystal (LC) material. An alignment layer or structure may be provided on the inner surface of each substrate, over the electrodes, to induce a desired uniform alignment of molecules of the LC material. Typically, electrodes on one surface may comprise rows of parallel conductive strips and electrodes on the other surface may comprise columns of parallel conductive strips at right angles to the rows. Picture elements (pixels) are defined by the overlap of row and column electrodes. When sufficient voltage is applied at a pixel, the LC material in the pixel is switched from its surface-aligned state to a different alignment state. The display includes means for distinguishing between the different states, for example one or more polarisers. In conventional nematic LCDs, the LC material reverts to the surface-aligned state when the electric field is removed from a pixel. Row and column electrodes are easy to manufacture, but conventional nematic liquid crystal displays require quite complex matrix addressing (multiplexing), and the number of pixels that can be addressed is limited.
An alternative to this passive matrix addressing is active matrix addressing, wherein each pixel is activated by a thin film transistor (TFT) which is part of an array. The transistor maintains the pixel in the required state until the display is next refreshed. A problem with active matrix displays is that large area TFT arrays are difficult to manufacture, particularly on polymer substrates.
Bistable displays offer a route towards high-complexity, high-quality, low-cost electronic displays. Each pixel can be switched either dark or light and will remain switched even after the applied voltage is removed. Complex displays with good contrast and viewing angle can be constructed without active matrix addressing. Known bistable displays use ferroelectric smectic LC materials. More recently, bistable displays have been developed which use nematic LC materials and microstructures to support two different LC alignments, for example as disclosed in EP 1 139 151 and EP 1 271 226. However, in order to be able to show full colour it is desirable that an LCD can display shades of grey.
It has been proposed in U.S. Pat. No. 4,712,877 to provide a ferroelectric LCD with greyscale by providing an insulating film on an inner surface of one cell wall, with ITO (indium-tin oxide) transparent electrodes on top of the film. The thickness of the film varies within a pixel so that the distance of the electrode on the film from a conventional planar electrode on the other cell wall varies. The ferroelectric LC has a threshold electric field above which switching occurs, but below which switching does not occur. The electric field strength experienced by the ferroelectric LC material for a given electrode voltage differs within the pixel so that at a lower applied voltage some areas of a pixel will experience an electric field above the threshold and be in an ‘on’ state and some areas will experience an electric field below the threshold and be in an ‘off’ state. At a higher applied voltage more or all of the pixel will be switched to the ‘on’ state. A problem with this approach is that the thickness of the LC layer varies, which compromises the optical performance of the display and ultimately limits the number of accessible grey levels. Another problem lies with the difficulty of forming ITO electrode structures on a film of varying thickness on a cell wall. Further problems are inherent in the use of ferroelectric materials, relating to the difficulty of obtaining uniform alignment in a robust display, and of obtaining a ferroelectric LC material with a sufficiently wide operating temperature range.
U.S. Pat. No. 5,257,122 describes an alternative ferroelectric LCD with greyscale capability. In this device, one of the cell walls is not uniformly planar and parallel with the other. One cell wall either has steps or a slope on which are formed electrode structures, while the other cell wall is planar and has planar electrodes. An alignment (orientation) layer is provided on the stepped or sloping electrode structures, in such amounts as to fill in the steps or gradient so as to present a substantially planar alignment surface to the layer of ferroelectric LC material, the alignment surface being parallel with the other cell wall. This device has a uniformly thick layer of LC material but suffers from the same problems inherent with all ferroelectric devices, and with the difficulty of manufacturing stepped glass and infilling with an alignment layer to provide a sufficiently smooth surface. The demonstration cell described in U.S. Pat. No. 5,257,122 is not a guide to practical device fabrication.